1. Field of the Invention
The present invention relates to a process for encoding and decoding binary data which serves to convert a binary data sequence into a binary code sequence suitable for recording or to convert a recorded binary code sequence into an original binary data sequence in recording or reproducing binary data into, or from a recording medium such as magnetic tape, magnetic disk, optical disk, or the like.
2. Description of the Prior Art
So far, various encoding systems have been proposed and utilized in order to enhance the recording density in recording binary data in a recording medium such as magnetic tape, magnetic disk, optical disk, or the like.
FIG. 1 is a diagram showing conventional binary data encoding systems. In FIG. 1, (a) represents a bit pattern of the original binary data sequence before the encoding, and the numerals 0 and 1 represent the logic "0" and "1" of the bit. In FIG. 1, the encoding system indicated by (b) is designated Modified Frequency Modulation System (MFM System), and it is used in magnetic disk apparatuses (type 3330, 3340, 3350, etc) of IBM. The encoding system indicated by (c) is designated (2, 7) Runlength Limited Code (RLLC), and it is used in type 3370 magnetic disk apparatus of IBM. The encoding system indicated by (d) in FIG. 1 is designated (1, 7) RLLC.
The encoding systems indicated by (c) and (d) in FIG. 1 are disclosed in the following articles: namely, with regard to (c), U.S. Pat. No. 3,689,899 entitled "Run-Length-Limited Variable Length Coding with Error Propagation Limitation", P. Franaszek, 1972: with regard to (d), Japanese Patent Laying-Open Gazette No. 128024/1977, entitled "Binary Data Coding System", invented by Toshio Horiguchi and filed by Nippon Electric Co., Ltd. According to the above mentioned articles, a recording current is applied to the recording medium to record the code sequence converted by each encoding system as the Non Return to Zero Inverse signal (NRZI signal), in which the polarity is inverted generally at the bit "1".
Now, the conversion algorism of the MFM shown in FIG. 1(b) will be shown in FIG. 2. The conversion algorism of the (2, 7) RLLC of FIG. 1(c) will be shown in FIG. 3, and the conversion algorism of the (1, 7) RLLC of FIG. 1(d) will be shown in FIG. 4.
In recording binary data in a magnetic medium or in an optical disk medium, the following conditions are required of a common encoding system:
(1) It is capable of high density recording in the recording medium;
(2) The error propagation in the decoded data is limited in the case where an error is generated in the transmission system (process of record reproduction);
(3) The amount of hardware is small in view of cost-effectiveness;
and so forth. The following parameters are used as practical parameters for estimation:
m: numbers of bits of a data word PA1 n: numbers of bits of an encoded word PA1 d: minimum number of "0" in a sequence of "0" PA1 k: maximum number of "0" in a sequence of "0" PA1 T: time interval of one bit of a data word PA1 Tw: detection window ##EQU1## Tmin: minimum interval between inversions . . . (2) Tmax: maximum interval between inversions . . . (3) PA1 Figure of Merit: ##EQU2##
In a common encoding system, the original data sequence is separated at every m bits to be converted into an encoded word of n bits. The converted code sequence is composed in such a manner that at least d "0"s but not more than k "0"s exist between a bit "1" and the succeeding bit "1".
In the case of high density recording of date in a recording medium, if the minimum interval between inversions (Tmin) become short, the conditions of recording transition (in magnetic recording, magnetic transition: in optical transition, recording pit) before and after the inversion are interfered with each other, causing errors in decoding a reproduction signal. If the detection window (Tw) is small, the number of errors in decoding increase due to various jitters in reproduction waveform, e.g. fluctuation or deterioration of the reproduction signal caused by an aberration in tracking the reproduction signal, fluctuation of the reproduction signal in media exchange, cross-talk between tracks, or external noise, or, particularly in an optical disk apparatus, distortion in the reproduced waveform derived from the astigmation fluctuation of the laser beams caused by an inclination of the disk, or asymmetry distortion of the pit caused by an aberration of the recording current. The production of the two parameters Tmin, Tw is designated as Figure of Merit, and the larger the value is, the smaller of the number of errors becomes, resulting in a high estimation. Meanwhile, in decoding the data, it is necessary to generate a clock for demodulation from the reproduced date, and, if the maximum interval between inversions (Tmax) is large, generation of the clock becomes difficult. Therefore, if the Tmax/Tmin ratio is small, the spectrum of the encoded code sequence centers upon the low frequency region, generally, and a distributed region of the spectrum becomes small, resulting in the enhancement of the reproduction S/N (Signal to Noise Ratio).
Table 1 shows the Figure of Merit of each of the encoding systems shown in FIG. 1.
TABLE 1 ______________________________________ system item MFM (2.7)RLLC (1.7)RLLC ______________________________________ FIG. of Merit 0.5 0.75 0.89 ______________________________________
As shown in FIG. 1, the largest Figure of Merit is obtained in the (1, 7) RLLC system. Namely, the (1, 7) RLLC is one example of an encoding system, having the following values: number of bits in a data word, m=2; number of bits in an encoded word, n=3; minimum number of "0" in a sequence of "0", d=1; maximum number of "0" in a sequence of "0", K=7; detection window, Tw=0.67 T; minimum interval between inversions, Tmin=1.33 T. The following theses have reported that in an encoding system, if said d and k are determined, the theoretical limit of Tw is determined, and that if d and Tw (Tw is determined by m and n) are determined, the theoretical limit of the value of k is determined.